This invention relates generally to blood processing systems and apparatus. More particularly, the invention relates to centrifuges for processing blood and, specifically, to a mount for supporting a thrust bearing at the middle of an umbilicus used in the fluid processing assembly of such a centrifuge.
Various blood processing systems now make it possible to collect particular blood constituents, rather than whole blood, from donors. Typically, in such systems, whole blood is drawn from a donor, the particular blood component or constituent is removed and collected, and the remaining blood constituents are returned to the donor. By thus removing only particular constituents, less time is needed for the donor's body to return to normal, and donations can be made at more frequent intervals than when whole blood is collected. This increases the overall supply of blood constituents, such as plasma and platelets, made available for health care.
Whole blood is typically separated into its constituents through centrifugation. This requires that the whole blood be passed through a centrifuge after it is withdrawn from, and before it is returned to, the donor. To avoid contamination and possible infection of the donor, the blood is preferably contained within a sealed, sterile system during the entire centrifugation process. Typical blood processing systems thus include a permanent, reusable centrifuge assembly containing the hardware that spins and pumps the blood, and a disposable, sealed and sterile fluid processing assembly that actually makes contact with the donor's blood. The centrifuge assembly engages and spins the fluid processing assembly during a collection procedure. The blood, however, makes actual contact only with the fluid processing assembly, which is used only once and then discarded.
To avoid the need for rotating seals, and to preserve the sterile and sealed integrity of the fluid processing assembly, blood processing systems often utilize centrifuges that operate on the "one-omega, two-omega" operating principle. This principle, which is disclosed in detail in Brown et al., U.S. Pat. No. 4,120,449, enables centrifuges to spin a closed system without the need for rotating seals and without twisting the components of the system. Blood processing systems that make use of the principle typically include a fluid processing assembly that includes a plastic bag that is spun in the centrifuge and that is connected to the blood donor through an umbilicus. The umbilicus is turned back on itself so that an end portion of the umbilicus is coaxially aligned with the axis of rotation of the bag. The intermediate portion of the umbilicus is twisted as the bag is spun to counteract the twisting that would otherwise take place as the bag is spun. The effect is that the end of the umbilicus, which is opposite the bag and is connected to the donor, does not twist as the bag is spun. The sealed, sterile integrity of the fluid processing assembly is thus maintained without the need for rotating seals.
U.S. Pat. No. 5,551,942 to Brown et al., commonly owned by the assignee hereof, discloses one such blood processing apparatus based on the "one-omega, two-omega" operating principle. In this apparatus, a disposable fluid processing assembly having an umbilicus and a processing chamber is mountable within a centrifuge assembly. On end of the umbilicus is held rotationally stationary substantially over the axis of centrifugation. The other end of the umbilicus joins the processing chamber and rotates with the processing chamber around the axis of centrifugation at the two-omega speed. The mid-portion of the umbilicus is supported by a wing plate that rotates around the axis of centrifugation at the one-omega speed. A thrust bearing mounted on the umbilicus permits the umbilicus to rotate relative to the wing plate as the wing plate and the processing chamber turn at different speeds. The thrust bearing slides into a one piece gimbal mounted in a recess provided on the wing plate. The gimbal helps keep the fluid processing assembly properly positioned during the centrifugation procedure. When the procedure is completed, the thrust bearing can be slid out of the gimbal in the wing plate to permit removal of the fluid processing assembly.
In prior fluid processing systems, it has proven difficult to achieve a reliable slide fit between the umbilicus thrust bearing and the one piece gimbal mounted in the recess in the wing plate. Ideally, the retaining forces developed between the thrust bearing and the gimbal should be great enough to reliably hold the thrust bearing against the forces developed during high speed centrifugation, but should not be so great as to distort the gimbal and thereby cause it to bind. This has required that the thrust bearing and the gimbal both be manufactured to very close tolerances. A thrust bearing that is slightly oversized physically distorts the gimbal thereby causing it to bind in the mounting recess. A slightly undersized thrust bearing results in excessive clearance and the possibility of inadvertent disengagement between the thrust bearing and the gimbal during operation. Additionally, the use of plastics in the manufacture of the umbilicus thrust bearing results in dimensional changes with changing humidity conditions. Thus, even when manufactured within the proper range of tolerance, a thrust bearing can still go out of tolerance with changing climatic conditions.